Tubular heating element

ABSTRACT

An electrical tubular heating element comprises an outer tube of metal and at least one electrical heating resistance, which is arranged in the outer tube. The zone adjacent to the inner wall of the outer tube is filled with magnesium oxide or another insulating material with a high melting point. Immediately adjacent to the heating resistance at least one layer or glass or another additional insulating material is provided so that when the temperature of the outer tube reaches a certain maximum value, a leakage current sufficient for switching off the tubular heating element flows between the coils of the heating resistance or a wire core arranged inside this heating resistance.

United States Patent 1 Bleckmann [54] TUBULAR HEATING ELEMENT I [76] inventor: Richard Bleckmann, Moserstrasse 29, Salzburg, Austria [22] Filed: May 17, 1971 [21} Appl. No.: 143,990

[30] Foreign Application Priority Data May 21,1970 Austria ..34535 [52] US. Cl. ..219/553, 219/335, 219/501, 219/517, 219/548, 338/238 [51] Int. Cl. ..H05b 3/10 [58] Field of Search ..219/546, 501, 504, 552-553, 219/505, 214, 318, 335, 517., 548; 338/238-242 m1 3,716,693 1 Fe b.13',1973

Primary Examiner-Velodymyr Y. Mayewsky Attorney-Stevens, Davis, Miller & Mosher [57] ABSTRACT An electrical tubular heating element comprises an outer tube of metal and at least one electrical heating resistance, which is arranged in the outer tube. The zone adjacent to the inner wall of the outer tube is filled with magnesium oxide or another insulating material with a high melting point. Immediately adjacent to the heating resistance at least one layer or glass or another additional insulating material is provided so that when the temperature of the outer tube reaches a certain maximum value, a leakage current sufficient for switching off the tubular heating element flows between the coils of the heatingresistance or a wire core arranged inside this heating resistance.

18 Claims, 9 Drawing Figures PATENTEDFEBII 3197s I SHEET 10F 4 TUBULAR HEATING ELEMENT The present invention relates to an electrical tubular heating element in which in a casing tube one or more heating resistances are accommodated which are insulated from the case tube by means of magnesium oxide or another high melting point insulating material and whose casing tube temperature may not exceed a maximum value chosen in accordance with the actual tube material and the purpose of use.

Such tubular heating elements are used in large numbers for heating liquids, more particularly water, for example in washing machines, dish washing machines or other liquid heaters or as heating elements in baking ovens, electric stoves, grilling devices etc. If, owing to some reason or other the removal of heat from the tube surface is hindered, for example, the build up of fur or the formation of foam in the case of washing machines or owing to running dry" due to control errors, such as in dish washing machines, the temperature in the interior of the tubular heating element rises to such a degree that the heating resistances begin to melt. This burning through of the tubular heating body can on the one hand occur in such a manner that an' arc, which is immediately extinguished, is only formed between the molten, ends of the heating resistance. In this case one speaks of a burning through towards the inside. It is also possible for an arc to be formed in the generally earthed outer tube, and in this case the tubular heating element casing cam often melt open along a long length. .ln the case of-AC-powered electrical tubular. heating elements for water heating, statistics show that in one third of the cases the burning through of the tubular heating element occurs towards the outside. In the case of radiant tubular heating elements with surface temperatures of above 700 C the proportion of tubular heating elements which have been burnt through towards the outside occurs in approximately two thirds of all cases. If the tubular heating element is surrounded bya liquid which has a greater or lesser high degree of conductivity, the melting open of the outer tube leads to the liquid penetrating into the interior of the tubular heating element so that an electrical connection is produced with the melted open end of the heating helix, which is still connected to the current, with the known dangers.

Owing to danger of fire it is also not possible to operate tubular heating elements in electrical apparatus which are provided with a plastics material container, without particular protective measures being taken. The necessary protective measures are, however, more expensive than the saving which is obtained by the use of a plastics material container.

The problem as to why tubular heating elements sometimes burn through to' the outside and some times to the inside has not, as far as is known, been systematically investigated in the literature. It is to be assumed that the metal vapors produced on the formation of an arc penetrate into cracks or pores of the insulating material and become deposited there until an electrically conducting bridge is formed with the outer casing and then a corresponding arc is produced with the outer casing.

It is object of the invention to construct a tubular heating body to obviate the overheating of the outer tube with its accompanying dangers.

- heating element exceeds a certain value.

It is another object of the invention to providea-tubular heating element which comprises at least one heating helix of resistance wire, in which at least one electrically conducting core is arranged which is insulated with respect to the heating helix with an insulating material layer, and this insulating material layer consists of a certain thickness and of a certain material so that a leakage current from the heating helix to the electrically conducting core occurs, when the external temperature of the tubular heating element casing increases above a certain value.

The invention is based upon the discovery that, providing certain dimensions are assumed, which, however, can be readily determined by one skilled in the art in accordance with a particular tubular heating element type and purpose of use, it is possible to produce a short circuit between the heating element coils or toa core wire arranged in the interior of the tubular heating element, when the temperature of the outer tube of the tubular heating element exceeds a certain determined value. In this respect it is of no importance whether this value is high or low. For certain applications, for example in the case of tubular heating elements, which are operated in the proximity of fusible or combustible plastics materials a tubular heating element temperature of as low as 300 C may not be permissible. in other cases, in the case of radiative heaters, a substantially higher temperature, for example 900 C, can be tolerated.

In accordance with the invention in addition tothe conventional high melting point insulating material, for example magnesium oxide use is made, immediately adjacent to the heating helix, of an'insulating material, for example glass, whose characteristic is so chosen that it becomes soft or molten and/or becomes electrically conducting when the temperature of the outer tube exceeds the permissible value. It is known that glasses with the most variable softening and melting points are available so that the technical objective can be realized in a large range of application.

if an electrically conducting core is used, which for example is connected with one end of the heating helix, there are in accordance with whether the heating coil is wound with a small or a large slope two effects which are to be distinguished from each other. If the coils of the heating helix are very close together, the additional insulating material melts over a large surface on overheating of the outer tube. There results owing to this melting a corresponding leakage current, which after reaching a certain level leads to actuation of an overcurrent device. After solidification of this molten material the heating element can be used again. If the coils of the heating element have, however, a greater distance from each other, there is in the case of an overheating at a localized area, a breaking through of the,additional insulating material. The metal vapors occurring in the case of the formation of the arc, become deposited in the path of the break-through and bring about a permanent short circuit. Such heating elements can never be used again. Since the additional insulating material is not used at a position adjacent to the heating helix and not adjacent to the outer tube, in every case it only comes to a short circuit between the coils of a helix or between the coils of several heating helices or between a heating coil and the core wire, which may be provided in the interior of the tubular heating element, but not with the outer tube.

The following description serves, in conjunction with the drawings, for further explanation of the subject matter of the invention. I

FIG. 1 isa section of the connection end of a tubular heating element with a conducting inner core.

FIG. la is a cross-section of the tubular heating body in'accordance with FIG. 1 along the line IaIa of FIG.

FIG. 2 is a section of a connection of a tubular heating body with a glass fiber fabric hose drawn over the heating helix.

FIG. 3 isa section of a connection end of a tubular heating element with a thickly wound heating helix.

FIG. 4 is a section through a container with a tubular heating element arranged in a vertical fashion, which serves as a check method. FIG. 5 is a section through a container with a tubular ,heating element arranged so as to be lying horizontally.

FIG. 6 is an illustrative diagrammatic view of a washing machine.

FIG. 7 is a section through an embodiment of a tubular heating body with a double inner core.

FIG. 8 is a section-through an embodiment of a tubular heating body which comprises two heating helices and which can be connected in different power stages.

In the drawings the tubular heating elements are shown on a somewhat enlarged scale. The actual dimensions generally correspond to the conventional dimensions of tubular heating bodies as are found in household devices.

Referring to FIGS. 1 to 3 thetubular heating bodies comprise a casing tube 1 which is made of metal, more particularly stainless steel and which is filled with magnesium oxide 2. Instead of magnesium oxide it is possible to use another refractory material. Since, however, numerousrequirements, more particularly as regards a good heat transfer, are to be made as regards theinsulating material, the selection is fairly small. In the-magnesium oxide 2 a heating resistance in the form of a heating helix 3 is embedded and a conducting inner core 5, the latter in the form of a piece of wire,'is inside the helix. A connecting bolt 6 has a reduced diameter part 7, on which the end of the heating helix is placed and which is connected'with the latter in an electrically conducting manner. A bead 8 of insulating material serves for closing the end of the tubular heating body or element. The other end of the tubular heating element is 'similarly'constructed. In the embodiments in accordance with FIGS. 1 and 3 the inner core 5 is not in electrical connection with the associated connecting bolt 6. The tubular heating'body itself can for example .be bent in the form of a U or in the form of a W.

Between the heating helix and the inner coil 5 there is a glass fiber fabric hose 9. The thickness of the glass fiber fabric and the melting point of the glass must, as appears from the later description, be selected in a certain manner to suit the permissible maximum temperature value permissible of the casing tube 1, so that a basic objective of the invention is fulfilled.

In order to compress the magnesium oxide the tubular heating element is generally mechanically pressed so that, as can be seen from FIG. la, it has an oval cross-section. It is essential that the zone 2a adjacent to the inner wall of the outer tube 1 exclusively contains the high melting point magnesium oxide. The embodiment shown in FIG. 2 differs from that shown in FIG. 1 in that instead of the inner core 5 and the glass fiber fabric hose 9 a glass fiber fabric hose 11 is provided which surrounds the heating helix 4, which has a lower angle of winding than the heating helix 3, and as a result, the individual coils are closer together.

In the case of the embodiment shown in FIG. 3 the heating helix 4 is arranged on a glass fiber hose 9, which surrounds a conducting inner core 5. The heating helix 4 itself is surrounded by means of a glass fiber fabric hose 1!. Its ends 40 are connected respectively with the reduced diameter portion 7 of the associated connecting bolt 6. The whole arrangement is, as is shown in the preceding figures, accommodated in a casing tube 1, into which magnesium oxide 2 is shaken.

If a tubular heating element in accordance with FIGS. 1 and 2 is overheated, for example owing to the voltage being increased in stages or owing to it not being possible for the heat produced to be conducted from the casing tube 1, at a specific temperature value of the casing tube 1 the glass fiber fabric hose 9 or the glass fiber fabric hose l1 begins to become soft or to melt. The melting temperature of the glass fiber fabric hose 9 or 11 is, as already mentioned, so chosen that in the zone lying adjacent to the heating helix this temperature is reached when the casing tube 1 reaches the permissible maximum temperature value. Close to the softening and melting temperature, however, the high specific resistance value of glass substantially decreases so that a substantial leakage current flows between the coils of the heating helices 3 or 4, and, if such is provided, to the inner core 5. This leakage current serves for a further heating of the glass melt so that a shortcircuit occurs. This shortcircuit ensures that the casing tube does not reach a temperature lying above the maximum permissible temperature value. Furthermore, it is reliably ensured that an arc is not formed extending fromthe heating helix 3 or 4 to the casing tube 1, by which the casing tube 1 would be melted open. In ac .cordancewith whether primarily this objectional burning through of the tubular heating element to the outside or only the overheating of the casing tube is to be avoided, preference is given to an embodiment in accordance with FIG. 1, FIG. 2, or the combined embodiment in accordance with FIG. 3.

If in the case of the embodiment in accordance with FIG. 1 the helix 3 is wound under tension on a glass fiber fabric hose 9 placed on the inner core 5, at the initiation of softening of the glass fiber fabric any one of the coils of the heating helix 3 will press the glass material to the side, so that a shortcircuit with the inner core5 is encouraged.

To explain the manner of operation in accordance with the embodiment shown in FIG. 2 the experimental arrangement in accordance with FIG. 4 is used.

In a container 12 a heating element 13, constructed in accordance with FIG. 2, is arranged vertically. The container 12 is filled with water, it being possible to lower the water from the level 14 over the level 15 to the level 16. The connection ends 17 of the tubular heating body 13 are arranged and the bottom of the container 12. As long as the water level is held at the level 14, there are normal operation conditions, as occur for example in the case of a liquid heater. If, in the case of the use of a conventional tubular heating element, for example owing to a control error, the water level sinks to the level 15, the end, extending above the level 15, of the tubular heating element 13 becomesred hot. If no special (and usually costly) safety measures are taken, the tubular heating body will burn through, and, as mentioned initially, in one third to two thirds for all cases, the burning will be towards the outside with a melting open of the casing tube. If the liquid level increases again, an electrically conducting connection is produced with the melted open end of the heating helix. Touching the water connections of the liquid heater can then be dangerous to life. In those cases in which the container 12 is made of plastics material, melting of the plastics material housing may take place owing to the radiation temperature. More particularly in the case of dish washing machines the plastics material housing may be burnt out. This has occurred again and again despite substantial safety measures being adopted.

On the other hand, with the tubular heating body constructed in accordance with the invention what happens is substantially different. If the water level sinks to the level 15, the temperature of the part, which is partially dry and lies above the water level 15, of the tubular heating element 13 rises, in the particular embodiment concerned, to 760 C. At this tubular casing temperature the conductivity of the glass fiber fabric hose 1] is so large that this part is substantially shortcircuited. The temperature dropped suddenly to room temperature. It was only shortly above the water level 15 that a piece, about 5 mm long, of the tubular heating element 13 continued to be red hot, because at this position in the interior the temperature was not yet sufficient for shortcircuiting. As a result of the shortened heated length of the tubular heating body the current consumption increased. After the water level sunk to the level 16, after some time the temperature of the part, extending above the water level 16, of the tubular heating element increased to 760 C and then the temperature of that part fell suddenly. At this moment an automatic safety device, connected in series, was actuated because the current strength had increased too much.

A burning through of the suitably constructed tubular heating element towards the outside was not observed in any of the numerous tests when the element became dry.

If a tubular heating element in accordance with FIG. 1

1 is used in a similar test arrangement, there then occurs in the case of the use of the same glass material a shortcircuit even at a lower temperature of the casing tube 1. There then follows a breakthrough at a localized position between the heating helix 3 and the inner core 5. After breaking through the duct remained conducting, apparently owing to condensed metal vapors, and the shortcircuit is therefore permanent.

0n the other hand, surprisingly, in the case of the embodiment in accordance with FIG. 3 in which a heating helix 4 is used with a low slope and consequently with a low heating surface loading of the wire, repeated use of the tubular heating body several times is possible. This is to be attributed to the fact that the glass fiber fabric hose 9 is not, as is the case with the embodiment in accordance with FIG. I only overheated at a localized position, that is to say at an abutment position of the heating helix 3, but a softening over an area occurs. Owing to the substantially larger softened surface there is also a greater leakage current which for example causes a fuse to respond before an arc-like breakthrough occurs. When the glass fiber fabric which had become soft hardens again the tubular heating body has substantially its former insulation value. It can be used several times again.

The manner of function naturally occurs when the glass fiber fabric hose 11 surrounding the heating helix 4 is omitted. FIG. 3 only serves to illustrate the fact that both measures, that is to say the conducting inner core 5 with the glass fiber fabric hose 9 and the glass fiber fabric hose l1 surrounding the heating helix can be combined in a single embodiment.

A local overheating of a tubular heating element can occur for a number of different reasons. In the case of washing machines for example a part of the heating element can-lie in current flow regions in which owing to eddy effects considerable foam formation occurs. In tubular heating elements which give up their heat to the air owing to radiation or convection, owing to hinderance in the air flow or other reasons the discharge of heat at a certain position of the tubular heating element can be worse than at the other position. Devices which normally operate perfectly under normal conditions, can in accordance with the manner in which they have been installed by the user, have different heat discharge characteristics. In the case of the use of conventional tubular heating elements the tubular heating element burns through sooner or later at the overheated position. In the case of a tubular heating element in accordance with FIG. 2 the overheated position is shortcircuited, while the tubular heating body continues to function normally along the remainder of its length.

If the tubular heating element after the shortcircuit is to be capable of functioning again, the surface loading of the resistance wire should be less than 20 to 25 watt per square centimeter, and preferably between 10 to 20 watt per square centimeter. In the case of tubular heating elements which cannot be switched on again, the surface loading is chosen so as to be between 25 and 40 watt per square centimeter or higher.

The thickness of the additional insulating layer, that is to say for example the glass fiber fabric hose, is to lie between 0.2 and 0.5 mm.

If a tubular heating body is used with an inner core in a liquid heater, then in accordance with a preferred embodiment of the invention, the heating coil with the inner core is to be electrically connected at the lower connection, so that at the upper connection there is the greatest potential difference between the heating helix and the inner coil. For explanation of this construction reference is made to FIG. 5.

A U-shaped bent tubular heating element 18, which has connections 19 and 21, which passes through a sealing body 22, is mounted in a container in such a manner that both its limbs run substantially horizontally one above the other and the connection 19 is arranged above the connection 21. At a position adjacent to the connection 21 the end of the heating helix 3 is directly placed on the inner core 5 and connected with it electrically, for example by welding. The associated connection 6 can consist of the inner core 5 continued outwardly. The end of the glass fiber fabric hose 9 does not extend quite as far as the connection bolt 6. At a position adjacent to the connection 19, however, the inner core 5 ends at a distance from the associated connection bolt 6. The drawn out end of the glass fiber fabric'hose 9 provides suitable insulation between the heating helix 3 and the inner core 5.

if now owing to drying up of the liquid level in the container from the normal level 24 to the level 25, the consequence is that a breakthroughoccurs between the heating helix 3 and the inner core 5 on the uppermost limb. Owing to the higher potential difference immediately adjacent to the connection 19 it is ensured that the limb of element 18 lying above the liquid cannot remain red hot for a long period of time with the accompanying dangers.

FIG. 6 shows a washing machine 26 with a drum 27 and a liquid sump container 28. A U-shaped bent tubular heating element 29 is arranged in a conventional manner in a horizontal position in the liquid sump container 28.

At the connection end at which the inner core is insulated from the heating helix the tubular heating element 29 is bent at 31 in the form of an offset in an upward direction. If the washing machine runs dry, the tubular heating body is shortcircuited at the position 31 as soon as the liquid level sinks below the height of this position. It is thus ensured that the remaining length of the tubular heating element does not become red hot.

In the case of the embodiment shown in FIG. 7 two inner cores 32, 33 are provided, which are insulated fromeach other by means of wound on glass silk or suitable glass hoses 34, 35, this insulation also serving to insulate the cores from the enclosing heating helix 3. The one end of each of the inner cores 32, 33 is connected with an associated connection bolt 6. The two ends 30 of the heating helix 3 are mounted on the reduced diameter parts 7 of the associated bolt 6. If now at any position in the tubular heating element overheating should occur, the glass layer between the two inner cores 32, 33 becomes conductive at this position. A safety device connected in series with the tubular heating element brings about immediate switching off of the tubular heating element. Since in the case of this arrangement the resistance wire of the heating helix 3 is generally not damaged and after solidification of the molten glass full insulation comes into being again, such a tubular heating element can be used repeatedly.

' of a material whose insulating resistance is low at high temperatures and only the layer, adjacent to the tube casing, of the insulating material is decisive for the amount of the leak currents, leads to new-possibilities as regards the construction of tubular heating elements. As is known it is very difficult to produce tubular heating elements with a high resistance by unit length. Generally in the case of tubular heating elements with 6 to 9 mm external diameter the possibility of production is limited-to approximately 600 ohms per meter. The reason for this is to be found in that in the case of such meter resistances it is necessary already to use resistance wires with a diameter of 0.15 mm, whose working in the helically wound form is very difficult. if any support or binding means are used for the helix, such means evaporate at the operating temperature so that the insulation value becomes poorer. in this connection use has already been made of asbestos for example with a corresponding binding material or means.

. However, in the case of the embodiments in accordance with the invention with an inner core it is possible to work substantially thinner resistance wires, since the resistance wire is held in position on the glass fiber fabric hose. If the winding is very tight, the material, giving way between the coils, of the glass fiber fabric hose provides suitable insulation between the individual coils. Furthermore, it is possible to wind even very thin resistance wires with a glass silk. In this manner it is possible to produce tubular heating elements with a very high ohmic resistance. The resistance wire wound with glass silk can, however, be substantially more easily worked than bare resistance wires. it is an especial advantage that these tubular heating elements with a high ohmic resistance do not burn through to the outside.

In the case of the embodiment of the invention shown in FIG. 8 of a tubular heating element two heating helices 36, 37 are provided which are each provided with glass fiber spun on material 42. The helix 36 is connected electrically with the connecting tag 38 and the heating helix 37 is connected electrically with the connecting tag 39. The other end of the heating helices 36 and 37 is mounted on a bolt 41. If these ends, as shown, are insulated off and the bolt 41 serves as a connecting bolt, then only the heating helix 36 or only the heating helix 37 or both heating helices 36 and 37 can be connected with voltage. This provides for a simple switching of power in this manner. If the ends remain insulated with respect to the bolt 41, then in the case of a certain length of the tubular heating element there is twice the resistance value and also the possibility of connection from one side. Although tubular heating elements which have a one-sided connection are already known, it is difficult to realize such constructions with the conventional external diameters 6 to 9 mm. This can be carried out, however, with the construction in accordance with FIG. 8 readily. Glass silk and glass fabric provide in the case of the embodiment in accordance with the invention for a very simple and easy workability. However, spun stone wool is also suitable. Furthermore, enamel layers, more particularly in the case of embodiments with inner cores, are suitable. The inner core then consists of a wire which consists of unalloyed steel which has a fired enameled coating. in this manner it is possible to achieve particularly low costs of production.

The finding is particularly surprising that the tubular heating elements described above have just as good an overall insulation resistance as normally constructed tubular heating elements. This is to be attributed to the fact that for the level of the insulation resistance in the case of tubular heating body the coldest zones of the insulating mass are decisive. Since also in the case of the ing an outer metal tube and at least one electrical resistance coil heating element having electrical terminals mounted and electrically insulated from the outer tube to define a heating zone therebetween; a high melting point first insulating material filling said zone, said first insulating material being refractory and heat conductive; an additional insulating material arranged immediately adjacent the heating element, said additional insulating material having a lower softening and melting point than said first insulating material and being more electrically conductive when so softened and melted, the point of softening and melting being so chosen that a fiow of current suitable to short circuit an over-heated portion of the resistance coil takes place through the additional insulating material when the temperature immediately adjacent to the heating element reaches a value at which the temperature of the outer tube exceeds the permissible maximum value.

2. A tubular heating element as claimed in claim 1, in which the additional insulating material is glass.

3. A tubular heating element as claimed in claim 1, in which the additional insulating material is a glass fiber fabric.

4. A tubular heating element as claimed in claim 1, in which the additional insulating material is a glass hose, whose wall thickness lies between 0.2 and 0.5 mm.

5. A tubular heating element as claimed in claim 1, in which the element is a heating helix in which at least one electrically conductive core is arranged, and is insulated from the heating coil by the additional insulating material.

6. A tubular heating element as claimed in claim 5, in which the electrically conductive core is formed by a wire extending substantially concentrically to the outer tube, the additional insulating material being arranged in a layer of a thickness of 0.2 to 0.5 mm with the coils of the heating element lying directly on the layer of the additional insulating material.

7. A tubular element as claimed in claim 6, in which on the electrically conducting core a glass fiber fabric hose is arranged with a thickness of 0.2 to 0.5 mm.

8. A tubular heating element as claimed in claim 1, in which the heating element is covered by a hose which consists of the additional insulating material.

9. A tubular heating element as claimed in claim 5, in which the electrically conductive core is coated with an enamel layer.

10. A tubular heating element as claimed in claim 1, in which the resistance wire of the heating element has spun round it a thread of the additional insulating material.

l l. A tubular heating element as claimed in claim 10,

in which the resistance wire of the heating element has a thread-like glass material spun round it.

12. A tubular heating element as claimed in claim 5, in which the electrically conductive core is connected with one of the two terminal ends of the heating element in an electrically conductive manner.

13. A tubular heating element as claimed in claim 5, in which at least one heating helix is provided, two parallel electrically conductive wire-shaped cores inside said at least one helix with the two core wires insulated from each other by means of the additional insulating material and a connection end of the heating helix is connected in an electrically conductive manner with one core and the other connecting end of the heating helix with the other core.

14. A tubular heating element as claimed in claim 10, in which several heating helices are provided in the form of a double or multiple helix with the resistance wires of the heating helices being insulated from each other by means of glass.

15. A tubular heating element as claimed in claim 1, in which the resistance wire is a helix arranged on a strand of the additional insulating material.

16. A tubular heating element as claimed in claim 1, in which the surface loading of the heating element is less than 25 watt per square centimeter so that it can be used again after actuating the overcurrent protection device.

17. A tubular heating element as claimed in claim 1, in which the additional insulating material is mineral wool.

18. The tubular heating element of claim 1 including insulating beads for closing the ends of the outer tube around the electrical terminals. 

1. In an electrical tubular heating element comprising an outer metal tube and at least one electrical resistance coil heating element having electrical terminals mounted and electrically insulated from the outer tube to define a heating zone therebetween; a high melting point first insulating material filling said zone, said first insulating material being refractory and heat conductive; an additional insulating material arranged immediately adjacent the heating element, said additional insulating material having a lower softening and melting point than said first insulating material and being more electrically conductive when so softened and melted, the point of softening and melting being so chosen that a flow of current suitable to short circuit an over-heated portion of the resistance coil takes place through the additional insulating material when the temperature immediately adjacent to the heating element reaches a value at which the temperature of the outer tube exceeds the permissible maximum value.
 1. In an electrical tubular heating element comprising an outer metal tube and at least one electrical resistance coil heating element having electrical terminals mounted and electrically insulated from the outer tube to define a heating zone therebetween; a high melting point first insulating material filling said zone, said first insulating material being refractory and heat conductive; an additional insulating material arranged immediately adjacent the heating element, said additional insulating material having a lower softening and melting point than said first insulating material and being more electrically conductive when so softened and melted, the point of softening and melting being so chosen that a flow of current suitable to short circuit an over-heated portion of the resistance coil takes place through the additional insulating material when the temperature immediately adjacent to the heating element reaches a value at which the temperature of the outer tube exceeds the permissible maximum value.
 2. A tubular heating element as claimed in claim 1, in which the additional insulating material is glass.
 3. A tubular heating element as claimed in claim 1, in which the additional insulating material is a glass fiber fabric.
 4. A tubular heating element as claimed in claim 1, in which the additional insulating material is a glass hose, whose wall thickness lies between 0.2 and 0.5 mm.
 5. A tubular heating element as claimed in claim 1, in which the element is a heating helix in which at least one electrically conductive core is arranged, and is insulated from the heating coil by the additional insulating material.
 6. A tubular heating element as claimed in claim 5, in which the electrically conductive core is formed by a wire extending substantially concentrically to the outer tube, the additional insulating material being arranged in a layer of a thickness of 0.2 to 0.5 mm with the coils of the heating element lying directly on the layer of the additional insulating material.
 7. A tubular element as claimed in claim 6, in which on the electrically conducting core a glass fiber fabric hose is arranged with a thickness of 0.2 to 0.5 mm.
 8. A tubular heating element as claimed in claim 1, in which the heating element is covered by a hose which consists of the additional insulating material.
 9. A tubular heating element as claimed in claim 5, in which the electrically conductive core is coateD with an enamel layer.
 10. A tubular heating element as claimed in claim 1, in which the resistance wire of the heating element has spun round it a thread of the additional insulating material.
 11. A tubular heating element as claimed in claim 10, in which the resistance wire of the heating element has a thread-like glass material spun round it.
 12. A tubular heating element as claimed in claim 5, in which the electrically conductive core is connected with one of the two terminal ends of the heating element in an electrically conductive manner.
 13. A tubular heating element as claimed in claim 5, in which at least one heating helix is provided, two parallel electrically conductive wire-shaped cores inside said at least one helix with the two core wires insulated from each other by means of the additional insulating material and a connection end of the heating helix is connected in an electrically conductive manner with one core and the other connecting end of the heating helix with the other core.
 14. A tubular heating element as claimed in claim 10, in which several heating helices are provided in the form of a double or multiple helix with the resistance wires of the heating helices being insulated from each other by means of glass.
 15. A tubular heating element as claimed in claim 1, in which the resistance wire is a helix arranged on a strand of the additional insulating material.
 16. A tubular heating element as claimed in claim 1, in which the surface loading of the heating element is less than 25 watt per square centimeter so that it can be used again after actuating the overcurrent protection device.
 17. A tubular heating element as claimed in claim 1, in which the additional insulating material is mineral wool. 